Magnetic lens arrangement

ABSTRACT

A highly compact magnetic lens arrangement which economically provides the highest field strength on the axis with the minimum beam half width and a minimum outer field strength of the coil winding comprises two tubular shielding cylinder means of superconductive material coaxially aligned with the lens axis, said cylinder means each having a first end and a second end, said first ends being spaced from each other to define a unshielded lens gap therebetween, said lens gap having a coil means positioned about the cylinder means to create a magnetic field, a cooling agent adapted to be present about the cylinder which cause a concentration of the magnetic field adjacent the particle beam, and a ferromagnetic ring-shaped pole shoe on each of said first ends of said cylinders for regulating and guiding the magnetic field.

United States Patent 1 Le Franc 1 Jan. 2, 1973 [54] MAGNETIC LENS ARRANGEMENT [75] Inventor: Guy J. Le Franc, Munich, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany [22] Filed: Nov. 30, 1971 211 Appl. No.: 203,283

[30] Foreign Application Priority Data Primary Examiner-George Harris Attorney-Carlton Hill et a1.

[57] ABSTRACT A highly compact magnetic lens arrangement which economically provides the highest field strength on the axis with the minimum beam half width and a minimum outer field strength of the coil winding comprises two tubular shielding cylinder means of superconductive material coaxially aligned with the lens axis, said cylinder means each having a first end and a second end, said first ends being spaced from each other to define a unshielded lens gap therebetween, said lens gap having a coil means positioned about the cylinder means to create a magnetic field, a cooling agent adapted to be present about the cylinder which cause a concentration of the magnetic field adjacent the particle beam, and a ferromagnetic ring-shaped pole shoe on each of said first ends of said cylinders for regulating and guiding the magnetic field.

5 Claims, 2 Drawing Figures MAGNETIC LENS ARRANGEMENT BACKGROUND OF THE INVENTION This invention generally relates to a magnetic lens arrangement for a particle or corpuscular beam device and more particularly concerns an objective lens arrangement for an electron microscope wherein two superconducting shielding cylinders disposed in coaxial relationship with the axis of the lens have a ring-shaped pole shoe on each of their ends. The shielding cylinders are disposed in a heat conductive relationship with a cooling agent. The cylinders have the effect of concentrating the magnetic field in the range of the corpuscular beam. The magnetic field may be produced by a coil or by a split coil having a suitable number of windings of superconductive material and carrying a suitable electric current. The lens gap between the inner ends of the shielding cylinders is free from any shielding and its size is chosen 'so that with a specific value (IL) of the field strength outside of the lens gap and outside of the cylinders the maximum value (H,,) of the field strength in the lens gap and the gradient of the field in the lens gap along the axis is of a value such that the constant of spherical aberration is below a predetermined value.

Prior art arrangements of the type referred to herein as shown for example, in the Offenlegungschrift No. 1,564,714 of the Federal Republic of Germany is concerned with a lens which is entirely free from any ferromagnetic material which would influence the magnetic field flow. In that patent the superconductive material of the screening cylinders prevents the magnetic field of the lens winding from having any effect on .the corpuscular or particle beams and the beams adapted to be focused by the lens arrangement at places other then the lens gap. This and other prior art constructions leave much to be desired. Thus, it is essential in attaining a high resolution power of the lens arrangement in an electron microscope to keep the spherical aberration as small as possible. This is obtained by keeping the so-called half width 2d, i.e. the field width at half the maximum value H,,, as low as possible with field strength (H,,) as high as possible to obtain a large field gradient in the lens gap. Making the shielding cylinders of a superconductive material allows a low half width. It is also true that the half width can be decreased by means of decreasing the gap width but this has the disadvantage that the maximum field strength (H decreases in the gap if H,, is not increased.

As is known, the beginning of the magnetic saturation of the material of the pole shoes strongly but disadvantageously influences the resolving power of the lens. This is particularly true in lens arrangements wherein the pole shoes are made of iron. In order to minimize this problem various ways have been tried. In one such attempt as reported in Comptes Rendus 1967, Series B, pages 747-750, the pole shoe tips were made of dysprosium or holmium. Another attempted solution described in Comptes Rendus 1969, Series B, pages 1707-1710, the lens arrangement included shielding and pole shoes made of iron but having superconductive cylinders arranged coaxially one behind the other in order to support the shielding by iron. Experiments have been made in order to avoid any ferromagnetic material for a high resolution lens arrangement because of the great mass and space requirements of the ferromagnetic material. Also, lens arrangements have been examined wherein exclusively superconductive material is used for shielding and shaping the magnetic field. However, despite this close attention to the problems in the prior art no solutions have emerged to the problem of providing a highly compact magnetic lens arrangement which economically provides the highest field strength on the axis with the minimum half width and a minimum outer field strength of the coil winding.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the deficiencies of the prior art and provide a means which allows an increase of the maximum field strength (H in the lens gap as economically as possible. More specifically, it is the object of this invention to provide a simple means for the efficient adjustment of the lens arrangement. These objects are met by the invention herein wherein a pair of coaxially aligned superconducting shielding cylinders having their inner ends spaced from each other each have on their inner end a ring-shaped pole shoe made of ferromagnetic material which cooperate to concentrate the magnetic flow in the lens gap. According to my invention the pole shoes are advantageously made of holmium or dysprosium which have ferromagnetic properties at low temperatures to give an essentially higher value of magnetic saturation or concentration than would be provided by iron, for example.

It was my discovery that the advantages of the high shielding effect of superconductive material could be utilized advantageously in combination with the concentric magnetic field effect of a ferromagnetic material having high saturation magnetizing such as is given with holmium and dysprosium. In the illustrated embodiment my invention comprises a ferromagnetic ring-shaped pole shoe on each of the first ends of two coaxially aligned tubular shielding cylinders wherein the pole shoes on the first ends are spaced from each other to define a lens gap therebetween. The magnetic field is created by current through a coil positioned about the cylinders. The pole shoe may have a generally frustoconical outer surface configuration.

In a further embodiment of my invention the surface of the shielding cylinder adjacent the first end may have an outer generally conical shape and the respective pole shoe for that cylinder end may have an inner conical surface of a larger included angle than the conical surface of the cylinder end so that a circular wedgeshaped gap is created between the inner surface of the pole shoe on the outer generally conical end surface of the cylinder. The pole shoes lie in a plane generally transverse to the lens axis, however, they are adjustable at an angle to the lens axis. The latter embodiment facilitates this adjustment.

It is a particularly advantageous feature of the invention herein that no space which might be utilized for the lens winding is lost near the lens gap because of the additionally provided ferromagnetic material. The lens arrangement according to this invention when compared to the prior art embodiment of the exclusively superconductive material will result in a higher field strength (H on the axis at a lesser half width with an equal outer field strength (H of the lens coil windings.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be efiected without departing from the spirit and scope of the novel concepts of the disclosure and in which:

FIG. 1 is a schematic cross sectional representation of a housing carrying the opposing shielding cylinders with the pole shoes at their inner ends and surrounded by a suitable magnetic field generating coil; and

FIG. 2 is an enlarged illustration of a further embodiment of a pole shoe on the tip of a shielding cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there may be seen a magnetic lens arrangement for a particle beam microscope according to the invention wherein a first tubular shielding cylinder 1 and a second tubular shielding cylinder 2 are coaxially aligned with a lens axis not shown. The first and second shielding cylinders l and 2 have inner end 1a, 2a, spaced from each other to define therebetween a free lens gap. Each of the first ends la,

20, receives thereon a ferromagnetic pole shoe 5, 6, respectively. The shielding cylinders 1, 2 are surrounded by an electrical coil which is split into two parts l1, 12, which consists of windings of a wire to create a magnetic field. The windings are of a superconductive wire of a size and number necessary to create the desired magnetic field. The shielding cylinders 1 and 2 are also of a superconductive material, especially of sintered Nb-Sn as is the outer material shielding housing generally indicated at 4. A liquid helium bath surrounds the parts ll, 12 of the lens coil as well as the shielding cylinders 1 and 2 so that the bores lb, 2b of the shielding cylinders 1 and 2 are free of magnetic fields.

The pole shoes 5 and 6 made of ferromagnetic material, preferably of holmium or dysprosium, are in heat conducting contact with the shielding cylinders 1 and 2, respectively, or the remaining cooled parts. With this arrangement the magnetic field of the coil is guided into the gap 9 between the ends la, 2a, of the first and second cylinders and highly concentrated. As a result of the high magnetizing concentration of the pole shoes 5 and 6 a high field strength (H,) with a low half width in the range of the axis of the lens may be obtained. For example, a field strength I-l equalling 60 kOe was obtained without difficulties with a half width 2d of 6 millimeters.

Adjustment of the precisely constructed lens arrangement is obtained bymeans of a shifting of the pole shoes which are provided according to this invention. This adjustment is facilitated since the pole shoes do not need to be positioned in-the liquid helium themselves. Because of this the gap 9 for the object to be inserted, which object has not been shown, is more easily accessible.

The enlarged cross sectional pole shoe construction of FIG. 2 illustrates a further embodiment of the invention having a particularly advantageous shaping of the pole shoe 16. In this embodiment a shielding cylinder illust ated t 22 has c nical su ace 5 t the e d which receives the pole shoe 16. T e po e s lroe in t is embodiment has an inner conical surface 17 and an outer surface 18. The included cone angle of the conical surface 15 is smaller than the included cone angle of the conical surface 17 so that a gap having a wedgeshaped ring configuration will result. As a result of this gap the adjustment of the pole shoe 16 is greatly facilitated. It will be understood that the surfaces 15 and 17 may be of other shapes such as spherical to create a gap therebetween. The corresponding shaping of the surfaces 15, 17 and 18 and the selection of the diameter of the bore 23 and/or 24, the path of the field in the gap may be varied.

Althoughminor modifications might be suggested by those versed in the art, it should be understood that I wish to embody within the scopeof the patent warranted hereon all such modifications as might reasonably and properly come within the scope of my contribution to the art.

I claim as my invention:

1. A magnetic lens arrangement for a particle beam microscope comprising two tubular shielding cylinder means of superconductive material coaxially aligned with the lens axis, said cylinder means each having a first end and a second end, said first ends being spaced from each other to define an unshielded lens gap therebetween, coil means positioned about the cylinder means to create a magnetic field, a cooling agent adapted to be present about the cylinders which cause a concentration of the magnetic field adjacent the par ticle beam, and a ferromagnetic ring-shaped pole shoe on each of said first ends of said cylinders for regulating and guiding the magnetic field.

2. A lens arrangement according to claim 1 wherein said pole shoe is made of a holmium material.

3. A lens arrangement according to claim 1 wherein said pole shoe is made of a dysprosium material.

4. A lens arrangement according to claim 1 wherein said pole shoes are arranged in a plane, said plane extending transverse to said lens axis and being adjustable.

5. A lens arrangement according to claim 1 wherein said first end of each of said shielding cylinder means has a generally conically shaped outer surface, said respective pole shoes for each of said cylinder ends having conical inner and outer surfaces said inner surface being generally larger than the conical angle of said outer surface of said first end so that a circumferential wedge-shaped gap is created between the inner surface of said pole shoe and said outer generally conical end surface of said cylinder. 

1. A magnetic lens arrangement for a particle beam microscope comprising two tubular shielding cylinder means of superconductive material coaxially aligned with the lens axis, said cylinder means each having a first end and a second end, said first ends being spaced from each other to define an unshielded lens gap therebetween, coil means positioned about the cylinder means to create a magnetic field, a cooling agent adapted to be present about the cylinders which cause a concentration of the magnetic field adjacent the particle beam, and a ferromagnetic ring-shaped pole shoe on each of said first ends of said cylinders for regulating and guiding the magnetic field.
 2. A lens arrangement according to claim 1 wherein said pole shoe is made of a holmium material.
 3. A lens arrangement according to claim 1 wherein said pole shoe is made of a dysprosium material.
 4. A lens arrangement according to claim 1 wherein said pole shoes are arranged in a plane, said plane extending transverse to said lens axis and being adjustable.
 5. A lens arrangement according to claim 1 wherein said first end of each of said shielding cylinder means has a generally conically shaped outer surface, said respective pole shoes for each of said cylinder ends having conical inner and outer surfaces said inner surface being generally larger than the conical angle of said outer surface of said first end so that a circumferential wedge-shaped gap is created between the inner surface of said pole shoe and said outer generally conical end surface of said cylinder. 